/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if (head == nullptr || head->next == nullptr)
            return head;
        ListNode *cur = head, *curnext = head->next, *cnt = curnext->next;
        head->next = nullptr;
        while (curnext) {
            curnext->next = cur;
            cur = curnext;
            curnext = cnt;
            if (cnt)
                cnt = cnt->next;
        }
        return cur;
    }
};